In transfer by a conventional fusion type thermal transfer printer, thermal heads, which are the heating elements, are aligned in one dimension (main scanning direction), and recording sheets or film are fed sequentially in the printing direction (sub scanning direction) over the thermal heads.
The heating amount of the thermal heads can be control led in steps, however, when melting and transferring color material, the thermal heads are easily affected by the dot density or adjacent dots, and it becomes difficult to control the gradation for each picture element, so control is performed using the two values: to melt and transfer/not to transfer. In this case, by drawing dots having a set size, gradation is expressed by area modulation. For example, a halftone generation method, which uses a rational tangent matrix, or a supercell type halftone generation method, which is based on this method and performs a pseudo increase of the number of gradations using a plurality of matrices, are used.
In addition to these methods, a method has been proposed in which the heating amount is changed according to the spatial arrangement of picture elements; for example, in Japanese patent application H8-20125, a technique is disclosed in which the dot area to be transferred as dots is controlled by the heating value of the heating elements, and in Japanese patent application H9-1866, a technique is disclosed in which a gradation conversion table is switched according to even lines and odd lines.